Embodiments of the invention relate generally to electrical power generators, and more particularly to running variable speed electrical power generators on a power grid.
In this regard, power grids operate near a set frequency, for example, 50 hz or 60 hz. The generators that produce the power often operate at a speed that is locked to the grid frequency. Generators that operate at a constant speed locked to the grid frequency are called synchronous generators.
Asynchronous generators operate at variable speeds and include a stator and rotor that are each controlled by a power converter. The power converters control electrical fields in the stator and rotor and adjust the fields to output a frequency that matches the grid frequency. As the speed of the generator changes, the power converters continually adjust the fields in the stator and rotor to match the grid frequency, so that the generator speed is not locked to the grid frequency.
One advantage of using asynchronous generators is realized when a gas turbine engine (engine) is used as a prime mover. When operating at maximum temperature, gas turbine engines increase mechanical power output when the speed of the gas turbine engine is increased. As the speed of the engine increases, the volume of air through the engine increases, and the engine may burn more fuel while respecting temperature limits, thereby producing more mechanical power. Using an asynchronous generator allows the gas turbine engine prime mover to operate at higher speeds than would a synchronous generator that is locked to the grid frequency.
While operating on a grid, if for example, other generators that supply power to the grid fail, the load on the generator will increase and the frequency of the grid may be reduced. Generators on the grid mitigate this frequency drop in two time frames. Because synchronous generators are locked to grid frequency, their speed will drop in response to the frequency depression. In the first fraction of a second, the energy stored in the inertia of synchronous machines is delivered to the grid as they slow down, and mitigates the rate of frequency decay, giving time for turbine controls to act to increase fuel. In the next several seconds the fuel increase in the turbine makes up for the generation deficit on the grid.
Introduction of asynchronous generators to the grid has the advantage of allowing more power from gas turbines by increasing their speed during a grid generation deficit, even though the grid frequency is depressed. However, the act of increasing speed draws power from the grid during the initial fraction of a second, to accelerate the inertia of the turgine-generator.
It is therefor the object of the present invention to define a strategy that accomplishes both an initial benefit to a grid disturbance by a speed reduction, and a longer-term benefit by a subsequent speed increase.